The present invention is concerned with a radiation power equalizer for optical communication systems. The invention also relates to a method of equalizing radiation power within such systems.
Optical communication systems conventionally communicate information by conveying information-bearing optical radiation having a wavelength in the order of 1.5 xcexcm. Such systems often now employ wavelength division multiplexing (WDM). When WDM is used, the optical radiation comprises one or more radiation components corresponding to channels where each component is modulated with associated communication traffic. Thus, the radiation is effectively partitioned into a discrete series of modulated radiation components.
Optical fibre waveguides incorporated into the communication systems are used to guide the radiation therearound. Attenuation occurring in the waveguides and associated optical devices connected thereto, for example add/drop multiplexers, necessitates the inclusion of optical amplifiers in series with the waveguides which improve signal-to-noise ratio by boosting radiation levels. Conventional optical amplifiers currently employed in optical communication systems are based on erbium-doped fibre amplification technology. Such amplifiers exhibit a non-linear amplitude-gain response and finite pumping power. As a consequence of this response, when radiation comprising a number of modulated radiation components is input to the amplifiers, it is important that the amplitudes of the radiation components are mutually similar otherwise the amplifiers will accentuate components of relatively greater radiation power in preference to components of relatively lesser power. Such accentuation becomes a serious issue when radiation passes through a number of cascaded erbium-doped fibre amplifiers (EDFAs) in the communication systems, especially when optical devices in the systems exhibit wavelength dependent transmission responses which accentuate certain radiation components more than others.
In conventional optical communication systems, matching of WDM radiation component amplitudes, namely channel leveling, is achieved by monitoring radiation component amplitudes at receiving locations where the radiation components are received and providing feedback instructions therefrom to transmitting locations where the radiation components are generated to adjust relative amplitudes of the components thereat so that the components are of similar power at the receiving locations. Such matching does not take into account variations in radiation component amplitudes at intermediate points in the systems between the transmitting locations and receiving locations at which EDFAs for radiation regeneration purposes are included. There therefore arises a potential problem that EDFAs at intermediate points can be supplied with radiation whose associated radiation components are not mutually similar in amplitude.
The inventors have appreciated that it is feasible to achieve channel leveling at the intermediate points in the systems using appropriately compact optical sub-systems capable of selectively attenuating or amplifying specific radiation components thereat. Moreover, the inventors have further appreciated that these aforementioned intermediate points can be add/drop multiplexers in a ring-based optical fibre waveguide network. Add/drop multiplexers are conventionally regarded as devices where diversion and addition of radiation components corresponding to specific channels is implemented, the diversion and addition of radiation components being achieved by way of switching operations using bi-state optical switching devices.
Thus, according to a first aspect of the present invention, there is provided a radiation power equalizer for an optical communication system, the equalizer characterized in that it includes:
(a) filtering means for partitioning information-bearing radiation received at the equalizer into one or more radiation components corresponding to wavelength division multiplexed communication channels of the system;
(b) liquid crystal attenuating means for selectively transmitting or attenuating said one or more radiation components;
(c) combining means for combining one or more of the radiation components transmitted or attenuated through the attenuating means to provide combined radiation;
(d) monitoring means for measuring power present in said one or more of the radiation components included in the combined radiation and generating one or more corresponding radiation component power indicative signals; and
(e) controlling means for receiving said one or more power indicative signals from the monitoring means and controlling attenuation provided by the attenuating means so that the radiation components included in the combined radiation are substantially of mutually similar power.
The invention provides the advantage that equalizer is capable of substantially mutually matching radiation power in radiation components output from the combining means so that such components subsequently do not cause power hogging effects in optical devices, for example in subsequent erbium-doped fibre amplifiers (EDFAs).
Such power hogging can cause diminution of relatively less powerful radiation components and accentuation of relatively more powerful radiation components, such disparity of power between radiation components potentially giving rise to increased data error rate and degraded signal-to-noise ratio.
In the context of the present invention, radiation components are substantially of similar mutual power when their respective powers are within a 20 dB error band, namely that the power difference between the weakest and most powerful radiation components in the radiation output from the combining means is not greater than 20 dB. In practice, the equalizer of the invention is capable of controlling radiation component power to within an error margin of 10 dB, preferably to within an error margin of 1 dB.
Thus, the controlling means is advantageously operable to mutually match the power of the one or more radiation components present in the combined radiation to within an error margin of 10 dB or less. Preferably, the controlling means is operable to mutually match the magnitudes of the one or more radiation components present in the combined radiation to within an error margin of 1 dB or less.
When implementing the equalizer in a communication system, it is beneficial that the filtering means is operable to partition the radiation received at the equalizer into spatially separated raylets, and the attenuating means preferably includes an array of liquid crystal cells, each cell corresponding to an associated raylet and operable to provide selective attenuation or transmission of the raylet. Partitioning the received radiation into spatially separated raylets eases the task of controlling the radiation power of each raylet selectively.
When the equalizer is included within an add/drop multiplexer of a communication system, it is advantageous that the equalizer includes dropping means for selectively diverting at least part of the one or more radiation components present in the radiation received by the filtering means, the part for output from the equalizer. Such diversion of at least part of the one or more radiation components enables the add/drop multiplexer to drop communication channels at the multiplexer, for example for conveying to clients connected to the add/drop multiplexer.
In a similar manner to the dropping means, the equalizer preferably includes adding means for adding additional radiation components to the combined radiation output from the combining means, the additional components and the combined radiation for output from the equalizer into the system. Inclusion of the dropping means and adding means enables the equalizer to both drop and add communication channels at the add/drop multiplexer into which it is incorporated. Conveniently, the controlling means is operable to control the adding means so that the additional radiation components and the one or more radiation components included in the combined radiation are mutually power matched to within an error margin of 10 dB; it is advantageous that the equalizer acts upon both radiation components transmitted through the attenuation means as well as components added through the adding means so that subsequent power hogging is unlikely to arise on account of the presence of the additional components.
When determining radiation power of radiation components within the equalizer for control purposes, it is advantageous that the monitoring means includes optical filtering means for partitioning radiation received thereat from at least one of the combining means and the adding means into radiation components, and an array of photodetectors for measuring radiation power present in each of the components and generating one or more corresponding output signals indicative of the radiation power present in the radiation components, the one or more output signals for use by the controlling means for controlling the attenuating means. There is thereby provided a parallel measurement of component radiation powers which can be implemented at relatively high speed but entails the relatively complex filtering means and the array of detectors.
Where relatively slower rate monitoring of radiation power of radiation components can be tolerated, it is preferable that the monitoring means includes tunable optical filtering means for serially selecting radiation components present in radiation received thereat from at least one of the combining means and the adding means, and a photodetector for measuring radiation power present in the selected series of radiation components and generating one or more corresponding output signals indicative of the radiation power present in the selected series, the one or more output signals for use by the controlling means for controlling the attenuating means. Such a serial manner of power measurement results in simplified hardware for the equalizer.
In operation, combining information-bearing radiation components of coincident wavelength inevitably results in data corruption within the equalizer. It is therefore beneficial that the attenuating means is operable to block radiation components propagating therethrough whose wavelengths are coincident with those of radiation components added to the combined radiation by the adding means.
Conveniently, for purposes of generating the additional radiation components, the adding means incorporates one or more modulated distributed feedback lasers for generating the one or more radiation components for adding to the combined radiation.
When implementing the equalizer, the filtering means associated with the attenuating means and the optical filtering means of the monitoring means are preferably implemented by using Bragg grating filters. Such grating filters can potentially provide relatively low cost compact practical assemblies.
When the equalizer is, for example, functioning as an add/drop multiplexer, it is preferable that the equalizer drops and adds appropriate amounts of radiation. In practice, it is found convenient that the equalizer is operable to divert in a range of 5 to 95% of radiation received thereat to the dropping means. Likewise, it is also convenient that the equalizer includes coupling means for coupling in a range of 5% to 95% of the combined radiation and in a range of 5% to 95% of the additional radiation into radiation for output from the equalizer to the system.
In a second aspect of the present invention, there is provided an add/drop multiplexer for an optical communication system, the multiplexer including a radiation power equalizer according to the first aspect of the invention.
In a third aspect of the present invention, there is provided a communication system including an add/drop multiplexer according to the second aspect of the invention.
According to a fourth aspect of the present invention, there is provided a method of equalizing radiation power in a radiation power equalizer, the method characterized in that it includes the steps of:
(a) receiving information-bearing optical radiation at the equalizer;
(b) partitioning the radiation into radiation components corresponding to communication channels;
(c) selectively attenuating one or more of the radiation components in liquid crystal attenuating means;
(d) combining radiation components transmitted or attenuated through the attenuating means to provide combined radiation;
(e) measuring the combined radiation using monitoring means to determine radiation power present in the one or more radiation components included in the combined radiation and generating one or more corresponding component radiation power indicative signals; and
(f) controlling attenuation provided by the attenuating means in response to the one or more power indicative signals so that radiation components present in the combined radiation are substantially of similar mutual power.
When controlling attenuation provided by the liquid crystal attenuating means, it is preferable to control such that, in step (f), the one or more radiation components present in the combined radiation are mutually power matched to within an error margin of 10 dB.